1. Field of the Invention
This invention relates to systems for locating, identifying and tracking objects. More particularly, it related to aircraft location, identification and docking guidance systems and to ground traffic control methods for locating and identifying objects on an airfield and for safely and efficiently docking aircraft at such airport.
2. Description of Related Art
In recent years there has been a significantly increased amount of passenger, cargo and other aircraft traffic including take offs, landings and other aircraft ground traffic. Also, there has been a marked increase in the number of ground support vehicles which are required to off load cargo, provide catering services and on going maintenance and support of all aircraft. With that substantial increase in ground traffic has come a need for greater control and safety in the docking and identification of aircraft on an airfield.
Exemplary of prior art systems which have been proposed for detecting the presence of aircraft and other traffic on an airfield are those systems disclosed in U.S. Pat. No. 4,995,102; European Patent No. 188 757; and PCT Published Applications WO 93/13104 and WO 93/15416.
However, none of those systems have been found to be satisfactory for detection of the presence of aircraft on an airfield, particularly, under adverse climatic conditions causing diminished visibility such as encountered under fog, snow or sleet conditions. Furthermore, none of the systems disclosed in the prior references are capable of identifying and verifying the specific type of an approaching aircraft. Still further, none of the prior systems provide adequate techniques for tracking and docking an aircraft at a designated stopping point such as an airport loading gate. Also, none of the prior systems have provided techniques which enable adequate calibration of the instrument therein.
The system disclosed in the above-cited parent application seeks to overcome the above-noted problems though profile matching. Light pulses from a laser range finder (LRF) are projected in angular coordinates onto the airplane. The light pulses are reflected off the airplane to detect a shape of the airplane or of a portion of the airplane, e.g., the nose. The detected shape is compared with a profile corresponding to the shape of a known model of airplane to determine whether the detected shape corresponds to the shape of the known model.
However, that system has a drawback. Often, two or more models of airplanes have nose profiles so similar that one model is often misidentified as another. In particular, in adverse weather, many echoes are lost, so that profile discrimination becomes decreasingly reliable. Since the models are similar but not identical in body configuration, a correct docking position for one can cause an engine on another to crash into a physical obstacle.
Thus, it has been a continuing problem to provide systems which are sufficiently safe and reliable over a wide range of atmospheric conditions to enable detection of objects such as aircraft and other ground traffic on an airfield.
In addition, there has been a long standing need for systems which are not only capable of detecting objects such as aircraft, but which also provide for the effective identification of the detected object and verification of the identity of such object, for example, a detected aircraft with the necessary degree of certainty regardless of prevailing weather conditions and magnitude of ground traffic.
There has also been a long standing, unfulfilled need for systems which are capable of accurately and efficiently tracking and guiding objects such as incoming aircraft to a suitable stopping point such as an airport loading gate. In addition, the provision of accurate and effective calibration techniques for such systems has been a continuing problem requiring resolution.
It will be readily apparent from the above that a need exists in the art for a more accurate identification of aircraft.
It is therefore a primary object of the invention to distinguish among multiple models of aircraft with identical or almost identical nose shapes.
It is a further object of the invention to improve the detection of aircraft so as to avoid accidents during aircraft docking.
To achieve the above and other objects, the present invention identifies aircraft in a two-step process. First, the profile matching is performed as known from the above-identified parent application. Second, at least one aircraft criterion matching is performed. In the aircraft criterion matching, a component of the aircraft, such as the engine, is selected as a basis for distinguishing among aircraft. The displacement of that component from another, easily located component, such as the nose, is determined in the following manner. An inner volume in which the engine is expected is defined, and an outer volume surrounding the inner volume is also defined. The LRF is directed at the inner and outer volumes to produce echoes from both volumes. A ratio is taken of the number of echoes in the inner volumes to the number of echoes in both volumes. If that echo exceeds a given threshold, the engine is determined to be present in the inner volume, and the aircraft is considered to be identified. If the identification of the aircraft is still ambiguous, another aircraft criterion, such as the tail, can be detected.
The aircraft criteria chosen for the second phase of the identification are physical differences that can be detected by a laser range finder. An example of such a criterion is the position, sideways and lengthwise, of an engine in relation to the aircraft nose. To consider an aircraft identified, the echo pattern must not only reflect a fuselage of correct shape. It must also reflect that there is an engine at a position, relative to the nose, where the expected aircraft does have an engine. Other examples of criteria that can be used are the position of the main gear, the position of the wings and the position of the tail.
The matching is preferably done only against the criteria specific for the expected aircraft type. It would be very time consuming to match against the criteria of all other possible types. Such matching would have to be against every type of aircraft that may land at a specific airport.
For each gate there is a defined a stopping position for each aircraft type that is planned to dock at that gate. There might be a safety risk for any other type to approach the gate. The stopping position is defined so that there is a sufficient safety margin between the gate and the aircraft to avoid collision. The stopping position for each aircraft type is often defined as the position of the nose gear when the door is in appropriate position in relation to the gate. There is a database in the system where the distance from the nose to the nose gear is stored for each aircraft type. The docking system guides the aircraft with respect to its nose position and stops the aircraft with its nose in a position where the correct type will have its nose gear in the correct stop position. If the wrong type is docked and if it has its wings or engines closer to the nose than the correct type, there is a risk of collision with the gate.
During the aircraft criteria phase, all aircraft criteria specified for the expected aircraft type can be checked. If an aircraft has a profile that can be used to discriminate it from any other type, which is rarely the case, the profile will be the only aircraft criterion. Otherwise, another criterion such as the position of the engine is checked, and if the identification is still ambiguous, still another criterion such as the position of the tail is checked.
The LRF is directed to obtain echoes from the inner and outer volumes. If the ratio of the number of echoes from within the inner volume to the number of echoes from within both volumes is larger than a threshold value, the aircraft is identified as having an engine at the right position, and that specific criterion is thus fulfilled. The ratio of the echo numbers is, however, just an example of a test used to evaluate the presence of an engine at the right position or to determine whether the echoes come from some other source, e.g., a wing. In cases in which that is the only criterion, the aircraft is considered to be identified. Otherwise, the other specified criteria (e.g., the height of the nose of the aircraft or evaluation of another aircraft criterion) have to be fulfilled.
If necessary, several characteristics, such as the tail, gears, etc., can be used to identify one specific type. The inner and outer volumes are then defined for each geometrical characteristic to be used for the identification. The exact extension of the volumes is dependent on the specific aircraft type and so is the threshold value.
A further identification criterion is the nose height. The nose height is measured so as to allow the horizontal scan to be placed over the tip of the nose. The measured nose height is also compared with the height of the expected aircraft. If the two differ by more than 0.5 m, the aircraft is considered to be of wrong type, and the docking is stopped. The value 0.5 m is given by the fact that the ground height often varies along the path of the aircraft which makes it difficult to measure with higher accuracy.
The invention lends itself to the use of xe2x80x9csmartxe2x80x9d algorithms which minimize the demand on the signal processing at the same time as they minimize the effect of adverse weather and bad reflectivity of aircraft surface. The advantage is that low-cost microcomputers can be used, and/or computer capacity is freed for other tasks, and that docking is possible under almost all weather conditions.
One important algorithm in that respect is the algorithm for handling of the reference profiles. The profile information is stored as a set of profiles. Each profile in the set reflects the expected echo pattern for the aircraft at a certain distance from the system. The position of an aircraft is calculated by calculating the distance between the achieved echo pattern with the closest reference profile. The distance interval between the profiles in the set is chosen so short that the latter calculation can be made using approximations and still maintain the necessary accuracy. Instead of using scaling with a number of multiplications, which is a demanding operation, simple addition and subtraction can be used.
Another important algorithm is the algorithm for determining an aircraft""s lateral deviation from its appropriate path. That algorithm uses mainly additions and subtractions and only very few multiplications and divisions. The calculation is based on areas between the reference profile and the echo pattern. As those areas are not so much affected by position variations or absence of individual echoes the algorithm becomes very insensitive to disturbances due to adverse weather.
The calibration procedure enables a calibration check against an object at the side of the system. The advantage is that such a calibration check is possible also when no fixed object is available in front of the system. In most cases, there are no objects in front of the system that can be used. It is very important to make a calibration check regularly. Something might happen to the system, e.g., such that the aiming direction of the system is changed. That can be due to an optical or mechanical error inside the system or it can be due to a misalignment caused by an external force such as from a passing truck. If that happens, the system may guide an aircraft to a collision with objects at the side of its appropriate path.
Another useful aspect of the present invention is that it can easily be adapted to take into account the yaw angle of the aircraft. The yaw angle is useful to know for two reasons. First, knowledge of the yaw angle facilitates accurate docking of the aircraft. Second, once the yaw angle is determined, the profile is rotated accordingly, for more accurate matching.
In the verification process it is determined whether certain geometric characteristics, such as an engine, are present in a certain position, e.g., relative to the nose. If the aircraft is directed at an angle towards the docking guidance system (DGS), which is often the case, that angle has to be known, in order to know where to look for the characteristics. The procedure is as follows:
1. Convert the polar coordinates (angle, distance) of the echoes to Cartesian coordinates (x,y).
2. Calculate the yaw angle.
3. Rotate the echo profile to match the yaw angle calculated for the aircraft.
4. Determine the existence of the ID characteristics.
The yaw angle is typically calculated through a technique which involves finding regression angles on both sides of the nose of the aircraft. More broadly, the geometry of the part of the aircraft just behind the nose is used. Doing so was previously considered to be impossible.
Still another aspect of the invention concerns the center lines painted in the docking ara. Curved docking center lines are painted as the correct path for the nose wheel to follow, which is not the path for the nose. If a DGS does not directly measure the actual position of the nose wheel, the yaw angle is needed to calculate it based on measured data, such as the position of the nose. The position of the nose wheel in relation to the curved center line can then be calculated.